First pass at implementing VotingPool.Withdrawal

Still lots of TODOs/XXXs and just a single (and overly long) test, but I've
been working on this too long now; it's time to get some feedback and start
tackling the remaining things step by step.

Oh, and this depends on https://github.com/gsalgado/btcscript/tree/rename-signTxOutput, which has already been submitted (but not yet accepted) upstream.
Since this is not supposed to be possible, I'd say that we need to abort the entire withdrawal request and return an 'internal error'. I think that goes for most of the 'what to do here'.